They are out there – we know they are – hiding in the remote, sodden bush of South Sudan’s Jonglei State. The state is vast, flat, and infuriatingly inaccessible, especially at this time of the year.

It is now the wet season, and not a single road is passable. Only a handful of airstrips are dry enough for planes to land, and the weather is so bad that even helicopters often struggle to fly. And motorised ground transport like four-wheel drives and motorcycles? Forget it. Swollen rivers and boggy ground make it tough to even walk across.

But a handful of people have walked, bringing with them disturbing accounts of a brutal ethnic conflict that has claimed hundreds – possibly thousands – of lives. They speak of heavily armed raiding parties attacking isolated villages, stealing cattle and killing everyone they find – women and children included – in a truly vicious cycle of killings, revenge and yet more revenge. There are stories of thousands of wounded people hiding in the bush, too afraid to come to government-controlled villages for help.

Their accounts paint a picture of a hidden war between the Murle and the Lou Nuer tribes, with allegations that the South Sudanese Army – the SPLA – is backing the Lou Nuer (allegations that the military vehemently denies). But the battlegrounds and killing fields are so remote and conditions so difficult, that it is almost impossible to get concrete evidence to support the claims, let alone find those still hiding in the bush.

The fighting is nothing new. The two communities have a long history of rivalry, often over land and water, but in recent months it all seems to have flared to disturbing levels of violence.

The French medical charity MSF reckons at least 100,000 people have been forced to run from their homes – a suspiciously round figure that seems to confirm the challenges of getting solid information. The victims seem as ethereal and intangible as ghosts, appearing only as frustratingly vague stories and rumours – never as real starving, wounded humans.

So, what really is going on?

We flew in to the town of Gumuruk, deep in Murle heartland, to try to find out.

The town is a desperately poor outpost with a single muddy street and a shabby market in the middle. An SPLA garrison has its headquarters in the schoolhouse. A contingent of UN peacekeepers is camped on the outskirts, MSF has set up a small medical clinic near the market and the World Food Program has an almost-empty canvas warehouse. And that’s it for international support.

There were rumours that some of the Murle survivors had settled a few hours walk away – close enough for women to venture in for help, but far enough to feel safe from the SPLA. We had hoped to find them with the help of a Murle guide, but the town’s government-appointed administrator refused to let us out.

“It’s too dangerous,” he told us. “The rebels have promised to kill anyone from the town, so I can’t let you go.” And it is true that the Murle rebel leader David Yau Yau has published a letter declaring any of his fellow tribesmen living in government-controlled towns to be “collaborators” and therefore “enemy”.

Still, the decision to go should be up to the Murle themselves. Not the administrator. And so we were left to talk to those who felt brave or desperate enough to come in from the bush themselves.

The horror-stories weren’t hard to find. Women (we could find no men who had walked in) told of raiding parties that had massacred all they could find. Rhoda said she lost all her seven children who where sheltering in the family boma while she was out collecting firewood; 14 of Luchia’s family were gunned down in a separate raid.

And while the accounts all had common elements – groups of Lou Nuer fighters storming a village or a boma, stealing cattle and killing people – we could find none that confirmed specific reports.

Once more, the victims appeared almost as phantoms; as characters in horror stories, tangible and believable, but without flesh and blood that we could confirm, see or touch.

So this crisis presents a deeply troubling dilemma for journalists, the aid community and international human rights organisations: how do you put pressure on donors and the South Sudanese government to help a crisis that you can’t see, let alone reach? How do you help people who are, to all intents and purposes, little more than ghosts?